When mounting semiconductor chips on a ceramics substrate on which a metallized layer made of a high-melting point metal is formed, the semiconductor chips and the metallized layer are electrically-connected by wire bonding. However, gold wire and aluminum wire cannot be wire-bonded directly on the metallized layer made of the high-melting point metal. So, the surface of the metallized layer is plated in advance and then wire bonding is carried out through the plating. The plating formed on the metallized layer, for example, has a structure in which an electroless nickel plating, a substitute gold plating, and an electroless gold plating are sequentially formed.
To connect semiconductor chips on the metallized substrate, it is necessary to dispose the semiconductor chips on the plating surface through gold tape and/or brazing filler metal and to treat them thermally. However, a nickel component derived from the base nickel plating tends to diffuse in the gold plating during the thermal treatment which tarnishes the surface of gold plating and which deteriorates connection strength of wire bonding.
As attempts to solve these problems, Patent document 1 discloses a process to inhibit diffusion of the nickel component to the gold plating layer by the steps of: forming a nickel plating layer on a metallized layer made of a high-melting point metal; crystallizing the nickel plating layer by heating; and gold plating on the nickel plating layer. Patent document 2 discloses that adhesiveness between a nickel plating layer and a gold plating layer can be improved by adjusting the temperature range of thermal treatment (sintering step).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 10-065297    Patent Document 2: JP-A No. 10-219469